The granting of a patent has often been described as a bargain [1] between the government and the patentee. In exchange for the exclusive right [2] to make, construct, use and sell their invention, the inventor will disclose the details of their discovery and, upon the patent's expiry, the invention will be made available to members of the public to use as they wish. For this reason, there is a prohibition [3] against obtaining multiple patents for the same invention, otherwise known as double patenting.

The validity of a patent can be challenged on the basis of double patenting. In Whirlpool, the Supreme Court noted that there were two branches of this doctrine. "Same invention"-type double patenting [4] asks whether the claims of the later patent are "identical or co-terminus" to an earlier patent, while "obviousness"-type double patenting asks whether a person skilled in the art would find the claims "patentably distinct". Earlier this year, two different federal court judges addressed allegations that Eli Lilly's '784 Patent was invalid due to double patenting of the invention in the earlier '377 Patent. (Mylan-Tadalafil [5] and Apotex-Tadalafil [6]). While both justices ruled in favor of Eli Lilly, they disagreed on the “appropriate date” at which the common general knowledge of a person skilled in the art should be assessed in order to undertake an obviousness-type double patenting analysis.

Both the '377 patent and the '784 patent relate to the compound tadalafil, which is now marketed by Eli Lilly under the brand name CIALIS and used in the treatment of erectile dysfunction (ED). However, it was not always known that tadalafil had such interesting uses, and the compound was initially developed as a treatment for disorders such as hypertension. Both Justice Gleason (Apotex-Tadalafil) and Justice de Montigny (Mylan-Tadalafil) agreed on the construction of the relevant claims in the '377 Patent (a claim for one compound, tadalafil, as a PDE V inhibitor) and in the '784 Patent (a claim for tadalafil as a PDE V inhibitor to treat ED). PDE V is an enzyme which catalyzes a chemical reaction that consumes a chemical that is responsible for, among other things, maintaining an erection. Tadalafil blocks this enzyme from functioning and thereby prevents the loss of an erection.

If, as of the "appropriate date", a person skilled in the art would have known that a PDE V inhibitor could be used to treat ED, then the two inventions would not be patently distinct and the '784 patent would be invalid for obviousness-type double patenting. The Court therefore needed to answer the question of what "appropriate date" should be used in the assessment.

In Mylan-Tadalafil the judge held that the "appropriate date" was the priority date of the earlier patent (January 21, 1994). At this point the evidence showed that the skilled person, although aware of the role of PDE V in cardiovascular disorders, would not have known about its role in erectile dysfunction. De Montigny rejected the priority date of the later patent (July 14, 1995) as the "appropriate date" for two reasons. First, such an analysis would simply become a question of whether the second patent was obvious [7] without the patentee being able to rely on the one-year applicant disclosure grace period.[8] Second, when considering "obviousness"-type double patenting, the question is whether the later patent "disclose[s] novelty or ingenuity" over the earlier patent. If it does, then a second patent is justified.

However, in Apotex-Tadalafil justice Gleason found there was a "sound argument" for the selection of the priority date of the second patent (July 14, 1995) as the "appropriate date". Choosing the later date would guard against a patentee abusing the 18 month delay between filing a patent application and it becoming available to the public. If the common general knowledge were to develop in such a way as to suggest an additional use for the patented compound (in conjunction with the confidential information in the patent application) the patentee would have the advantage of filing a second patent that would have been obvious had the 18 month delay not been in place.

In the end, the issue became moot as the only piece of prior art published between the two dates (the '902 application above) turned out not to be relevant. However, if the issue were to be decided, I find the reasoning of Justice de Montigny more compelling. While the scenario described by Justice Gleason could occur, the patentee would run the risk of a competitor independently discovering the information in the first patent and filing an application for the new use themselves.

[1] Pioneer Hi-Bred Ltd v Canada (Commissioner of Patents), [1989] 1 SCR 1623.
[2] Patent Act, RSC, 1985, c P-4 s. 42.
[3] Whirlpool Corp v Camco Inc, 2000 SCC 67 at para 63. [Whirlpool]
[4] Whirlpool, at paras 64-65.
[5] Eli Lilly Canada Inc v Mylan Pharmaceuticals ULC, 2015 FC 17. [Mylan-Tadalafil]
[6] Eli Lilly Canada Inc v Apotex Inc, 2015 FC 875. [Apotex-Tadalafil]             
[7] Patent Act, RSC, 1985, c P-4 s. 28.3. For the Supreme Court of Canada's approach to the non-obviousness requirement for a patent see Apotex Inc v Sanofi-Synthelabo Canada Inc, 2008 SCC 61.
[8] Patent Act, RSC, 1985, c P-4 s. 28.3(a). For a summary of the one-year grace period for applicant disclosure see here.